Pressurized fluidized bed combustion system including control of temperature of flue gases entering a high temperature filter

ABSTRACT

A pressurized fluidized bed combustion system includes a pressure vessel, a furnace section within the pressure vessel for forming high temperature, high pressure gases and for generating steam, a compressor for providing compressed combustion air for the furnace section, a filter for cleaning the high temperature, high pressure flue gases formed in the furnace section, a gas turbine for generating power from the gases formed in the furnace section and cleaned in the filter, a steam turbine cycle for generating power from the steam generated in the furnace section, a heat recovery section, connected to the steam turbine cycle, for recovering heat from the gases exhausted from the gas turbine and for supplying the heat to feedwater provided for the steam generation in the furnace section, and an inlet duct for the filter. The inlet duct is connected between a gas discharge channel in the furnace section and a gas inlet channel in the filter. The inlet duct includes a first gas duct, a second gas duct, a first branch, connecting the gas discharge channel in the furnace section with the first and second gas ducts, for dividing the flow of gases from the furnace section into a first gas flow and a second gas flow, a second branch, connecting the first and second gas ducts with the gas inlet channel in the filter, for recombining the first and second gas flows and a gas cooler in the second gas duct for decreasing temperature of the second gas flow.

This application is a divisional of application Ser. No. 08/807,805,filed Feb. 26, 1997 (U.S Pat. No. 5,953,898).

BACKGROUND AND SCOPE OF THE INVENTION

The present invention relates to a method of operating a flue gascleaning process and a flue gas filtering system utilizing filterssusceptible to damage at temperatures exceeding a characteristic T_(max)for the filters and including means for preventing such damage of thefilters. The present invention also refers to a power generation methodand a pressurized fluidized bed combustion system utilizing the flue gasfiltering system.

The present invention more specifically relates to controlling thetemperature of hot gases before leading them to a ceramic filter forparticle removal. A need to control the temperature of hot gases existsparticularly in combined cycle processes which utilize a pressurizedfluidized bed combustor, and when there is a ceramic filter between thecombustor and a gas turbine, which ceramic filter is easily damaged,when exposed to too high of temperatures.

In combined cycle processes, solid and liquid fuels can be utilized withhigh thermal efficiency. Thereby, fuel is combusted at, e.g, a 12-16 barpressure in a fluidized bed combustor, whereafter the hot flue gases areexhausted from the combustor and allowed to expand in a gas turbine. Inorder to protect the gas turbine vanes from erosion and fouling, theflue gases are carefully cleaned before being introduced into the gasturbine. The cleaning is typically carried out in a gas purifier systemincluding ceramic filters.

Ceramic filters are very proficient for separating fine solid particles,such as ash and sorbents, from the hot gases. It has, however, turnedout that ceramic filters are highly sensitive to too high of gastemperatures at which the crystal structure of the material may changeor its bonding agent may deteriorate, resulting in the filters becomingfragile and being broken. This has become a major problem. It isimportant that precautions are taken to avoid deterioration of ceramicfilters, which are very expensive to repair, since the breaking of evena single filter tube in a combined cycle process can cause extensivedamage to the gas turbine. Even small damage in the gas filter can becostly, as the whole plant may have to be stopped for removing a singledamaged filter element, which easily may take several days.

The manufacturers of ceramic filters today usually state a highestrecommended temperature for using the filters, which should--as aminimum precaution--be carefully followed. Typically, the combustionconditions are chosen so that a suitable flue gas temperature isobtained at the filter inlet. It would, however, in many cases be betterif additional separate means were available for adjusting the filterinlet temperature. Given the fact that ceramic filter materialstechnology is still under development and the maximum allowable gastemperature at the inlet of a filter may in many cases be unknown, itwould be desirable to have means of controlling the filter inlettemperature with a variable set point.

The thermal efficiency of combined cycle processes, such as pressurizedcirculating fluidized bed (PCFB) combined cycle processes, improves withincreasing combustor flue gas temperatures, i.e., improves when thetemperature of flue gases entering the gas turbine is increased.Therefore, it is not economical to cool the flue gases to a too low of atemperature level before introducing them into the ceramic filter. Thus,instead of cooling the flue gases considerably, it is recommendable toadjust the flue gas temperature such that the temperature remains at anas high of a temperature level as possible, but still below temperaturesabove which the life of the filter material will be significantlyshortened or failure could result. These conflicting requirements,related to thermal efficiency and delicacy of the filter material,require that it is essential to maintain the inlet temperature belowT_(max), but as close to T_(max) as possible.

The possible fluctuations of the flue gas temperature, which may occur,e.g., due to variations of the fuel quality, make it necessary to leavea temperature margin, when adjusting the temperature of the flue gases.The temperature margin, between the aimed flue gas temperature and thehighest recommended inlet temperature of the ceramic filter, has to beof the order of the amplitude of the largest expected temperaturefluctuations of the flue gases. As such a temperature margin lowers thethermal efficiency, there is clearly a need for an as accurate and asrapid as possible flue gas temperature control means which can suppressthe flue gas temperature fluctuations to 10° F., preferably 5° F. oreven less. No such easily controllable adjustment means has until nowbeen available.

British patent GB 2,261,831 discloses a method and an apparatus forprotecting ceramic filter elements from sudden temperature variations byusing heat storing elements to stabilize the temperature of flue gasesbefore allowing them to come into contact with the ceramic filterelements. This method is not applicable for adjusting the temperature ofthe flue gases, only for damping its fluctuations.

European patent publication EP 0 611 590 suggests the introduction of anincombustible powder into flue gases prior to filtration, when a rise influe gas temperature is foreseen. By this method, undesired temperaturerises may be suppressed, but as a side effect, clogging of thefiltration apparatus is accelerated.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an improved method ofoperating a flue gas cleaning process and an improved flue gas filteringsystem, in which the temperature of hot gas at an inlet to a ceramicfilter may be adjusted for thus protecting the filter from being exposedto too high of temperatures.

Another object of the present invention is to provide an improved methodof operating a flue gas cleaning process and an improved flue gasfiltering system for increasing the thermal efficiency of a combinedcycle process including a combustor and a ceramic filter whilesimultaneously maintaining an acceptable inlet temperature for theceramic filter.

A further object of the present invention is to provide an improvedmethod of operating a flue gas cleaning process and an improved flue gasfiltering system for increasing the average flue gas temperature of acombustor in a combined cycle process which includes a ceramic filterdownstream from the combustor.

A still further object of the present invention is to provide animproved method of operating a flue gas cleaning process and an improvedflue gas filtering system for providing an accurate and rapid control oradjustment of the temperature of hot gases before they are brought intocontact with a ceramic filter.

It is also an object of the present invention to provide an improvedmethod of operating a flue gas cleaning process and an improved flue gasfiltering system for controlling the flue gas temperature of a combustorwith a control procedure having a variable set point, to be used, e.g.,in studying the effect of flue gas temperature changes on the durabilityof ceramic filters.

It is further an object of the present invention to provide an improvedpower generation method, in a combined PFCB cycle process utilizingceramic filter elements to clean flue gases prior to the gas turbine,with increased thermal efficiency.

It is still further an object of the present invention to provide animproved pressurized fluidized bed combustion system, which includesceramic filter elements for flue gas cleaning and in which damage to theceramic filter elements due to high flue gas temperatures is minimized.

Toward the fulfillment of these and other objects, the method ofoperating a flue gas cleaning process of the present invention forcleaning high temperature flue gases in a filter, comprises the steps of

(a) providing a flow of high temperature flue gases to be cleaned;

(b) dividing said flow of high temperature flue gases into a first flowof flue gases V₁ and a second flow of flue gases V₂ ;

(c) decreasing the temperature of said second flow of flue gases, forproviding a cooled flow of flue gases;

(d) combining said first flow of flue gases and said cooled flow of fluegases, for providing a recombined flow of flue gases, and

(e) leading said recombined flow of flue gases into said filter.

The temperature of the second flow of flue gases is preferably decreasedby passing said second flow of flue gases over heat exchange surfaces ina gas cooler section, e.g., by leading the second flow of flue gases ina vertical path through a duct having vertical heat exchange tubestherein.

The ratio V₁ /V₂ of the first flow of flue gases V₁ to the second flowof flue gases V₂ may be controlled by controlling the first flow of fluegases V₁ or alternatively by controlling the second flow of flue gasesV₂. The recombined flow of flue gases may then be adjusted to atemperature in the range T_(max) to (T_(max) minus 30° F.), preferablyT_(max) to (T_(max) minus 10° F.), most preferably T_(max) to (T_(max)minus 5° F.), by controlling the ratio V₁ /V₂ of the first flow of fluegases V₁ to the second flow of flue gases V₂ or by controlling thetemperature of the second flow of flue gases.

An inlet ducting, for a flue gas filtering system utilized for cleaninghigh temperature flue gases in a filter, such as a ceramic filter,susceptible to damage at temperatures exceeding a characteristic T_(max)for said filter, comprises according to the present invention

(a) a flue gas channel for providing a flow of high temperature fluegases;

(b) a first flue gas duct connected to said flue gas channel;

(c) a second flue gas duct connected to said flue gas channel;

(d) a first branching connecting said flue gas channel to said first andsecond flue gas ducts, for dividing said flow of high temperature fluegases into a first and second flue gas flow;

(e) an inlet channel, for introducing flue gas to said filter;

(f) a second branching connecting said first and second flue gas ductsto said inlet channel, for recombining said first and second flue gasflows in said inlet channel to a recombined flow of flue gases, and

(g) gas cooling means in said second flue gas duct, for decreasing thetemperature of said second flue gas flow.

The system preferably also includes means for controlling the firstand/or second flue gas flow, such as a valve arranged in the firstand/or second flue gas duct. A valve arranged in the second flue gasduct is preferably arranged downstream of the gas cooling means therein.The flue gas ducts are preferably mainly vertical and gas cooling meansused to decrease the temperature of the flue gas flow preferablyincludes vertical heat exchange tubes, such as water tubes, arrangedparallel with the flue gas flow within the duct.

The system according to the present invention also preferably includesmeans for measuring the temperature of the recombined flow of flue gasesand means for controlling the ratio V₁ /V₂ of the first flow of fluegases V₁ and second flow of flue gases V₂, for controlling thetemperature of the recombined flow of flue gases, to provide arecombined flow of flue gases having a temperature within a temperaturerange T_(max) to (T_(max) minus 30° F.), preferably (T_(max) to T_(max)minus 10° F.), most preferably T_(max) to (T_(max) minus 5° F.).

The improved power generation method according to the present inventioncomprises the steps of

forming a furnace section in an enclosure;

fluidizing a bed of combustible material in said furnace section andcombusting said combustible material in said furnace section;

discharging flue gases and fine particulate material entrained thereinfrom said furnace section;

introducing discharged flue gases and fine particulate materialentrained therein into a filter, such as a high temperature ceramicfilter;

separating said fine particulate material from said flue gases in saidfilter, for cleaning said flue gases;

passing the thus cleaned flue gases to a power generating section;

allowing said cleaned gas to expand in a gas turbine in said powergenerating section, and

controlling the temperature of flue gases discharged from said furnacesection before introducing said flue gases into said filter by

(a) dividing said flow of high temperature flue gases into a first flowof flue gases V₁ and a second flow of flue gases V₂ ;

(b) decreasing the temperature of said second flow of flue gases in agas cooler section, for providing a cooled flow of flue gases;

(c) combining said first flow of flue gases and said cooled flow of fluegases, for providing a recombined flow of flue gases to be introducedinto said filter.

According to a preferred power generation method according to thepresent invention, there is provided a fluid flow circuit includingwater tubes forming heat exchange surfaces in the gas cooler section andwater/steam tubes in said furnace section, absorbing heat from thecombustion of combustible material, and a steam turbine for powergeneration. The feed water for the water/steam tubes in the furnacesection is heated in the gas cooler section.

The improved pressurized fluidized bed combustion system according tothe present invention comprises:

a pressure vessel;

a furnace section within said pressure vessel for forming hightemperature high pressure gases and steam;

a compressor for providing compressed combustion air for the furnacesection;

a filter for cleaning said high temperature high pressure flue gasesformed in the furnace section;

a steam turbine cycle for generating power of said steam formed in thefurnace section;

a gas turbine for generating power of high temperature high pressuregases formed in the furnace section and cleaned in the filter;

a heat recovery section, for recovering heat from exhaust gases from thegas turbine, and

an inlet ducting for the filter, the inlet ducting being connectedbetween a gas discharge channel in the furnace section and a gas inletchannel in the filter, the inlet ducting further comprising

(a) a first gas duct;

(b) a second gas duct;

(c) a first branching connecting said gas discharge channel in thefurnace section with said first and second gas ducts, for dividing saidflow of high temperature high pressure gases into a gas flow and asecond gas flow;

(d) a second branching connecting said first and second gas ducts withsaid gas inlet channel in the filter, for recombining said first andsecond gas flows to a recombined flow of gases, and

(e) gas cooling means in said second gas duct, for decreasing thetemperature of said second gas flow.

The present invention is especially applicable in combined cycleprocesses which utilize a pressurized fluidized bed combustor, whenthere is a ceramic filter between the combustor and a gas turbine, whichceramic filter is easily damaged when exposed to too high oftemperatures.

The present invention provides an easy and reliable way of controllingthe temperature of hot flue gases from a combustor or similar hot gassource before introducing the gas into the ceramic filter, whilemaintaining an as high as possible gas temperature. The hot flue gasesfrom the combustor are, according to a preferred embodiment of thepresent invention, passed to the ceramic filter through a flue gas ductor channel, which includes a section which is split into a first channeland a second channel, the second channel being parallely connected tothe first channel and having heat exchange surfaces therein. The fluegas stream from the combustor is thereby divided--with a controllabledivision ratio--to two sub-streams, one of which is led through thefirst channel and the second of which is led through the second channel.The first sub-stream is according to a preferred embodiment of theinvention uncooled, whereas the second sub-stream is cooled by heatexchange surfaces in the second channel. The first and second flue gassub-streams are recombined after the cooling of the second sub-stream.The thus formed recombined gas flow is then led into the ceramic filterto be purified therein before being introduced into the gas turbine.

According to a preferred embodiment of the present invention, the inlettemperature to the ceramic filter is controlled by means of a valve,such as a butterfly valve, controlling the division ratio of the fluegases between the first channel and the second channel. The butterflyvalve may be inserted in the first channel for controlling the flow ofuncooled flue gases or in the second channel for controlling the flow ofcooled flue gases. Either way the ratio of uncooled to cooled flue gasflow and thereby the temperature of the recombined gas flow may becontrolled.

It would according to another embodiment of the present invention bepossible to control the temperature of the recombined gas flow bycontrolling the heat exchange in the second channel.

The temperature of the recombined flue gas flow is according to thepreferred embodiment of the present invention adjusted by measuring thetemperature of the flue gases entering the ceramic filter and thenadjusting the division ratio of the flue gas stream such that thetemperature of the recombined flue gas flow meets the predeterminedtemperature, e.g., T_(max) to (T_(max) minus 30° F.), preferably T_(max)to (T_(max) minus 10° F.), most preferably within about T_(max) to(T_(max) minus 5° F.), where T_(max) is the maximum temperature allowedfor gas entering the ceramic filter.

The means for controlling the temperature of gas at the inlet to theceramic filter thereby according to the preferred embodiment of theinvention also includes temperature measuring means, such as athermometer, for measuring the temperature of the flue gases enteringthe ceramic filter and controlling means, for controlling the butterflyvalve and thereby the divisional ratio of flue gas flows throughdifferent channels on the basis of the measured flue gas temperature.

The heat exchange surfaces in the second channel act according to apreferred embodiment of the present invention as final feed waterheaters heating feed water for the steam drum connected to the boiler(combustor) steam/water system. Feed water from the steam turbinecondenser is typically heated in the gas turbine's exhaust gas heatrecovery section before being passed through the final feed waterheater, i.e., the flue gas cooler, into the steam drum. The feed watermay additionally be passed through an air heater located between the gasturbine's heat recovery section and the gas cooler for heating thecombustion air.

The changing of the set point for gas temperature at the ceramic filterinlet does not in this arrangement according to the present inventionsubstantially affect the feed water temperature, as

an increase in water heating at the ceramic filter inlet leads to asimultaneous decrease in gas temperature and a decrease in heat recoveryat the gas turbine's heat recovery section, and on the other hand

a decrease in water heating at the ceramic filter inlet leads to asimultaneous increase in gas temperature and an increase in heatrecovery at the gas turbine's heat recovery section.

Therefore, by utilizing the present invention, the flue gases to be ledto the ceramic filter can be maintained at an optimal temperature, i.e.,without substantially affecting the steam generation and withoutdamaging the ceramic filter tubes.

BRIEF DESCRIPTION OF THE DRAWINGS

The above brief description, as well as objects, further features andadvantages of the present invention will be more fully appreciated byreference to the following detailed description of the presentlypreferred but nonetheless illustrative embodiments of the presentinvention when taken in conjunction with the accompanying drawingswherein:

FIG. 1 is a schematic representation depicting a typical arrangement ofa combined cycle process with a pressurized combustor in accordance withthe present invention,

FIG. 2 is a schematic representation depicting a vertical cross sectionof an inlet channel to a ceramic filter according to an embodiment ofthe present invention, and

FIG. 3 is a schematic representation depicting a vertical cross sectionof an inlet channel according to another embodiment to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates a power plant which is designed tooperate on coal or the like fuel and comprises combined steam and gascycles. The illustrated system includes a pressure vessel 10 with apressurized circulating fluidized bed combustor (PCFB) or boiler 12therein. The PCFB combustor produces hot pressurized gas, which is usedto generate power in a gas turbine 14, and steam which is used togenerate power in a steam turbine 16.

The gas turbine 14 runs a generator 18 for generating electrical powerand a compressor 20 for supplying pressurized air to the PCFB combustor12 by way of a supply line 22. An air heater 24 is provided for heatingthe air before introduction of it into the pressurized vessel 10. Fromthe vessel, air is fed into the combustor 12 through the bottom thereoffor fluidizing the bed therein and for providing combustion air.

The walls of the combustor 12 are typically water tube walls, lined withevaporating tubes 26 converting water to steam. Feed water is introducedto the tubes via a steam drum 28 connected to the upper part of thecombustor. Steam generated in the tubes is recirculated to the steamdrum, whereafter steam is conducted via line 30 to the combustor forsuperheating in means not shown. Superheated steam is passed throughline 32 to the steam turbine 16. The steam turbine 16 runs a generator34 which generates electrical power.

A mixture of coal or other fuel and limestone or other sorbents is fedinto the combustor and maintained in a fluidized state by means of thecompressed air, which is supplied thereto from the compressor 20. As thefuel is burned, hot flue gases are produced. Larger particles entrainedin the hot flue gases are usually first separated in a particleseparator (not shown in detail) whereafter the partly purified hot fluegases pass through a conduits 36 and 36' to a ceramic filter 38. A fluegas inlet temperature controller 40 is provided in the conduit 36' priorto the ceramic filter 38. The temperature controller allows the setpoint for the desired flue gas temperature to be adjusted according toneed. The ceramic filter 38 removes substantially all remaining fineparticles from the hot flue gases and passes the cleaned hot gases alonga gas line 42 to the inlet 44 of the gas turbine.

The gas turbine 14 is connected to a heat recovery section 46, whereheat is recovered from the gas turbine exhaust gases the gases beforeare exhausted through the stack 48.

A steam condenser 50 is connected to the steam turbine 16 outlet forproviding feed water. The feed water is heated in the heat recoverysection 46 before being cooled in the air heater 24 and re-heated in thehot gas inlet temperature controller 40 and recirculated to the steamdrum 28.

Control means 52 is connected to an appropriate temperature monitoringmeans 54, such as a thermometer, for monitoring the temperature of thehot flue gases in line 36' between the temperature controller 40 and theceramic filter 38. The control means 52 is further connected to a means(not shown in FIG. 1) for controlling the flue gas division ratio withinthe inlet temperature control system of the ceramic filter 38. Theconstruction of the temperature control system is shown in more detailin FIGS. 2 and 3.

FIGS. 2 and 3 provide a schematic illustration of two embodiments of theflue gas inlet temperature controller 40 according to the invention. Thesame reference numerals are used in FIGS. 2 and 3 where applicable.

The inlet temperature controller 40 includes an inlet duct 56 fromconduit 36, in gas flow communication with the combustor 12, and anoutlet duct 58 to conduit 36' in gas flow communication with the ceramicfilter 38. The temperature control system 40 further comprises twoparallel gas channels 60 and 62. Channel 62 includes a gas coolersection 64 with heat exchange surfaces 66. In the embodiment shown inFIG. 2, the channel 60 includes means 68 for controlling the gas flowthrough this channel and thereby controlling the division ratio of theflue gas flows between the two channels. The means 68 for controllingthe gas flow through channel 60 may be a valve, such as a butterflyvalve, or any other suitable means for providing a variable flowresistance in said channel.

In the embodiment shown in FIG. 3, the corresponding means 68 is locatedin channel 62 downstream of the gas cooler section 64. It may benecessary to arrange a flow restrictor in channel 60, or some othermeans in the inlet ducting, for accomplishing a gas flow through bothchannels 60 and 62.

A hot flue gas flow 70 from the combustor 12 (shown in FIG. 1) flowsthrough the inlet duct 56 into the temperature controller system and ahot flue gas flow 72, however, in most cases, having a slightly lowertemperature is discharged via the outlet channel 58. The temperature ofthe discharged gas flow 72 is adjusted in the temperature controller toa desired level, close to the maximum temperature level allowed to passinto the ceramic filter.

Typically, the method and system according to the present inventionshould be capable of controlling a filter inlet gas temperature byseveral tens of degrees Fahrenheit in a time scale of some tens ofseconds, e.g., the system may decrease or increase a flue gastemperature with>10° F., preferably>30° F., or even>60° F. in about 10to 30 seconds. Commercial ceramic filter materials are currentlyavailable with a highest recommended operation temperature of about1650° F. A typical set point for the flue gas at the ceramic filterinlet is currently somewhat below 1650° F. but will most probably in thefuture be above 1650° F.

The temperature of the gas flow 72 at the outlet 58, i.e., of the gasflowing into the ceramic filter inlet, can be regulated by adjusting theposition of the butterfly valve 68. This adjustment provides a change inthe percentage of gas flowing over the heat exchange surface 66 inchannel 62 and a change in the temperature of the recombined flow ofgas.

The heat exchange surfaces 66 comprise one or more vertical gas flowpaths 74 over vertical water tubes 76, the vertical surfaces 66 and flowpaths 74 preventing fine solid particles from accumulating on heatexchange surfaces. Thus, a need for soot blowers in the pressurizedenvironment is eliminated.

In FIGS. 2 and 3, the flue gas cooler is described as having waterflowing inside heat transfer tubes 76 and gas on the outside thereof,which may be the preferred solution for high pressure steam cycles. A"fire-tube" design, with the flue gas flowing through tubes, which arearranged in a tube package covered by a shell or jacket, and the waterflowing between the tubes inside the shell or jacket, may be a morepreferred solution in low pressure steam cycles.

A feed water inlet pipe 78 and a feed water outlet pipe 80 are connectedto the gas cooler section 64. The feed water inlet pipe 78 is, as shownin FIG. 1, connected through the air heater 24, heat recovery section 46and condenser 50 to the steam turbine 16 outlet side. The feed wateroutlet pipe 80 is on the other hand connected to the steam drum 28.

The gas cooler section 64 and the means 68 for controlling the divisionratio of gas flow between the two channels 60, 62 may be located indifferent channels, as shown in FIG. 2, or in the same channel, as shownin FIG. 3.

If the flow division controlling means 68 and the gas cooling means 64are located in the same channel 62, as shown in FIG. 3, then thedivision controlling means 68 should preferably be located downstream ofthe gas cooler section 64, for preventing the division controlling meansfrom being contacted with flue gases at their highest temperatures.

The area of the heat exchange surfaces in the gas cooler section 64 hasto be designed such that the flue gas stream can, at all times, becooled with sufficient efficiency, i.e., to be able to suppress allharmful temperature fluctuations of the flue gases. The maximum controlrange of the system is obtained when the ratio of the flue gases cominginto contact with the heat exchange surfaces can be varied from zero toone. In this respect, the best possible result is obtained by having acontrollable flow impedance, such as a butterfly valve, in both of theparallel channels 60 and 62.

The inlet temperature controller 40 may be located within the mainpressure vessel 10 of the fluidized bed combustor or on the outsidethereof.

There is no intention to limit the invention to the above exemplaryembodiments, but on the contrary, it is intended to be applied andmodified within the scope of protection defined in the accompanyingpatent claims.

I claim:
 1. A pressurized fluidized bed combustion system comprising:a pressure vessel; a furnace section within said pressure vessel for forming high temperature, high pressure gases and for generating steam; a compressor for providing compressed combustion air for said furnace section; a filter for cleaning the high temperature, high pressure flue gases formed in said furnace section; a gas turbine for generating power from the high temperature, high pressure gases formed in said furnace section and cleaned in said filter; a steam turbine cycle for generating power from the steam generated in said furnace section; a heat recovery section, connected to said steam turbine cycle, for recovering heat from gases exhausted from said gas turbine and for supplying the heat to feedwater provided for the steam generation in said furnace section; and an inlet duct for said filter, said inlet duct being connected between a gas discharge channel in said furnace section and a gas inlet channel in said filter, said inlet duct comprising (i) a first gas duct, (ii) a second gas duct, (iii) a first branch, connecting said gas discharge channel in said furnace section with said first and second gas ducts, for dividing the flow of high temperature, high pressure gases from said furnace into a first gas flow and a second gas flow, (iv) a second branch, connecting said first and second gas ducts with said gas inlet channel in said filter, for recombining the first and second gas flows into a recombined flow of gases; and (v) gas cooling means, in said second gas duct, for decreasing the temperature of the second gas flow.
 2. A pressurized fluidized bed combustion system according to claim 1, further comprising a heat exchanger connected to said steam turbine cycle between said heat recovery section and said furnace.
 3. A pressurized fluidized bed combustion system comprising:a pressure vessel; a fluidized bed boiler within said pressure vessel for combusting combustible material and for forming high temperature, high pressure gases; a filter for cleaning the high temperature, high pressure gases formed in said fluidized bed boiler; and an inlet duct, connected between a gas discharge channel of said fluidized bed boiler and a gas inlet channel of said filter, for leading a flow of the high temperature, high pressure gases from said fluidized bed boiler to said filter, said inlet duct comprising (i) a first gas duct, (ii) a second gas duct, (iii) a first branch, connecting said gas discharge channel with said first and second gas ducts, for dividing the flow of high temperature, high pressure gases from said fluidized bed boiler into a first flow and a second flow of gases, (iv) a second branch, connecting said first and second gas ducts with said gas inlet channel of said filter, for recombining the first and second flows of gases into a recombined flow of gases, and (v) gas cooling means, in said second gas duct, for decreasing the temperature of the second gas flow.
 4. A pressurized fluidized bed combustion system according to claim 3, further comprising temperature monitoring means, in the gas inlet channel of said filter, for monitoring the temperature of the recombined flow of said high temperature, high pressure gases prior to entry into said filter.
 5. A pressurized fluidized bed combustion system according to claim 3, further comprising control means for controlling the division of the flow of high temperature, high pressure gases from said fluidized bed boiler into a first flow and a second flow of gases.
 6. A pressurized fluidized bed combustion system according to claim 5, wherein said control means comprises means for controlling the gas flow in said first gas duct.
 7. A pressurized fluidized bed combustion system according to claim 6, wherein said means for controlling the flow in said first duct comprises a valve.
 8. A pressurized fluidized bed combustion system according to claim 5, wherein said control means comprises means for controlling the gas flow in said second gas duct.
 9. A pressurized fluidized bed combustion system according to claim 8, wherein said means for controlling the flow in said second gas duct comprises a valve.
 10. A pressurized fluidized bed combustion system according to claim 9, wherein said valve is arranged downstream of said gas cooling means.
 11. A pressurized fluidized bed combustion system according to claim 3, wherein said second gas duct is a generally vertical duct and said gas cooling means comprises vertical heat exchange tubes arranged parallel to the gas flow within said vertical second gas duct.
 12. A pressurized fluidized bed combustion system according to claim 11, wherein said heat exchange tubes comprise water tubes.
 13. A pressurized fluidized bed combustion system comprising:a furnace section for fluidizing a bed of combustible material to combust the combustible material in said furnace section; a discharge for discharging a flow of high temperature flue gases and fine particulate material entrained therein from said furnace section; a flue gas temperature controller for controlling the temperature of flue gases and fine particulate material entrained therein discharged from said furnace section by said discharge; a duct for introducing the flow of high temperature flue gases and the fine particulate material into said flue gas temperature controller; a gas flow divider, in said duct, for dividing the flow of high temperature flue gases and the fine particulate material into a first flow of flue gases V₁, and a second flow of flue gases V₂ in said flue gas temperature controller; a gas cooler section for decreasing the temperature of the second flow of flue gases in said flue gas temperature controller, to provide a cooled flow of flue gases; a combining duct for combining the first flow of flue gases and the cooled flow of flue gases in said flue gas temperature controller to provide a recombined flow of flue gases; a high temperature ceramic filter for filtering the recombined flow of flue gases and the fine particulate material entrained therein; an inlet for introducing the recombined flow of flue gases and the fine particulate material entrained therein into said high temperature ceramic filter; means for measuring the temperature of the hot flue gases and the fine particulate material at a position prior to entry into said filter; and a controller for controlling the temperature of the flue gases entering said filter by adjusting the flow of high temperature flue gases and the fine particulate material in said flue gas temperature controller, on the basis of the temperature measured by said measuring means.
 14. A pressurized fluidized bed combustion system according to claim 13, further comprising (i) a separator for separating the fine particulate material from the flue gases in said filter to clean the flue gases and (ii) a turbine supply for passing the thus cleaned flue gases to a gas turbine in a power generating section to cause the cleaned flue gases to expand in the gas turbine.
 15. A pressurized fluidized bed combustion system according to claim 13, further comprising means for decreasing the temperature of the second flow of flue gases in said flue gas temperature controller by passing the second flow of flue gases through a gas cooler section having a plurality of aligned heat exchange surfaces therein.
 16. A pressurized fluidized bed combustion system according to claim 15, further comprising a fluid flow circuit that includes (i) water tubes, which form the heat exchange surfaces in said gas cooler section, (ii) tubes in said furnace section, which absorb heat from the combustion of the combustible material, and (iii) at least one steam turbine, which generates power.
 17. A pressurized fluidized bed combustion system according to claim 16, wherein feed water in the tubes in said furnace section absorbs heat in said gas cooler section after having been heated in a heat recovery section arranged in a gas flow downstream of a gas turbine.
 18. A pressurized fluidized bed combustion system according to claim 13, wherein said controller further comprises means for controlling the ratio V₁ /V₂ of the first flow of flue gases V₁ to the second flow of flue gases V₂ by controlling the flow rate of at least one of the first flow of flue gases V₁ and the second flow of flue gases V₂ in said flue gas temperature controller.
 19. A pressurized fluidized bed combustion system according to claim 13, further comprising regulating means for regulating the temperature of the recombined flow of flue gases to a temperature in a range of T_(max) to (T_(max) minus 30° F.), wherein T_(max) is the maximum temperature allowed for gas entering the filter.
 20. A pressurized fluidized bed combustion system according to claim 19, wherein said regulating means comprises means for controlling the ratio V₁ /V₂ of the first flow of flue gases V₁ to the second flow of flue gases V₂ in said flue gas temperature controller.
 21. A pressurized fluidized bed combustion system according to claim 19, wherein said regulating means comprises means for controlling the temperature of the second flow of flue gases in said flue gas temperature controller.
 22. A pressurized fluidized bed combustion system according to claim 13, further comprising regulating means for regulating the temperature of the recombined flow of flue gases to a temperature in a range of T_(max) to (T_(max) minus 10° F.), wherein T_(max) is the maximum temperature allowed for gas entering the filter.
 23. A pressurized fluidized bed combustion system according to claim 22, wherein said regulating means comprises means for controlling the ratio V₁ /V₂ of the first flow of flue gases V₁ to the second flow of flue gases V₂ in said flue gas temperature controller.
 24. A pressurized fluidized bed combustion system according to claim 22, wherein said regulating means comprises means for controlling the temperature of the second flow of flue gases in said flue gas temperature controller.
 25. A pressurized fluidized bed combustion system according to claim 13, further comprising regulating means for regulating the temperature of the recombined flow of flue gases to a temperature in a range of T_(max) to (T_(max) minus 5° F.) wherein T_(max) is the maximum temperature allowed for gas entering the filter.
 26. A pressurized fluidized bed combustion system according to claim 25, wherein said regulating means comprises means for controlling the ratio V₁ /V₂ of the first flow of flue gases V₁ to the second flow of flue gases V₂ in said flue gas temperature controller.
 27. A pressurized fluidized bed combustion system according to claim 25, wherein said regulating means comprises means for controlling the temperature of the second flow of flue gases in said flue gas temperature controller.
 28. A pressurized fluidized bed combustion system according to claim 14, wherein said gas turbine drives an air compressor.
 29. A pressurized fluidized bed combustion system according to claim 28, wherein said compressor compresses air, and further comprising a supply for supplying the compressed air to said furnace section to support combustion of the combustible material. 